Preheat your oven to 350°F. In a double boiler or microwave, melt the chocolate and butter together until smooth. Remove from heat and whisk in the granulated sugar until the mixture becomes light and fluffy. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Sift in the flour, cocoa powder, and salt, then gently fold the dry ingredients into the wet mixture until just combined. Pour the brownie batter into a greased baking pan and smooth the top with a spatula. Evenly sprinkle the crushed graham crackers over the batter, then add a generous layer of marshmallows on top. Bake for 25 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. For extra toasted marshmallow flavor, finish with a brief broil or use a kitchen torch to lightly toast the marshmallows. Allow the brownies to cool completely before cutting into squares.